My invention relates to a line isolation monitor and, more particularly, to a circuit for monitoring the level of electrical leakage current between the lines of an isolated AC electrical system, which are isolated from electrical ground potential, and electrical ground potential. My invention also relates to a novel voltage level alarm system particularly useful in my novel line isolation monitor.
The electrical line isolation monitor provides the means of detecting an unduly high leakage of electrical current between the lines of an isolated electrical system and electrical ground potential or "ground" from which that electrical system is ideally isolated. As is known, electricity is supplied by the utility company and is distributed to on-premises locations by at least two electrical lines. Typically one of the two lines is connected to electrical ground potential and this thus creates a voltage difference between the other electrical line and any object or equipment connected to "ground". In some applications an isolated or "ungrounded" electrical system must be employed, such as in hospital operating rooms. In this application it is necessary to eliminate any difference of potential or leakage current path between the electrical supply system and ground. The reasons for this is at least two-fold: One is to eliminate the danger of electrical shock to a patient undergoing connection directly or indirectly to various types of medical electronic equipment; the second is to avoid the generation of electrical sparks particularly in locations where combustible gases, used as anesthetics, are employed and which may explode in the presence of an electrical spark.
Conventially, hospital electrical systems employ what is known as an electrical isolation transformer, a transformer which contains a primary winding and a separate secondary winding of approximately the same number of turns in each winding in which there is no direct electrical connection beween the windings. The primary winding is connected to the electrical lines supplied by the electrical utility, one of which is grounded, and the secondary winding, which is isolated from direct electrical contact with the primary, is connected via a pair of electrical lines through the various electrical outlets in the hospital location. Thus a person could hold one end lead of the secondary winding in one hand and touch electrical ground potential with the other hand without receiving any electrical shock. Inasmuch as the primary, which is connected to ground, and secondary are isolated from one another, there is no voltage difference between the aforedescribed locations.
In practice, however, there is always some finite amount of leakage current, however small, for example, between the secondary winding, through the electrical insulation to ground; between the secondary and primary or between any part of the winding or electrical leads, through the electrical insulation to ground. In a practical system these leakage paths possess impedance levels on the order of "mogohms" of resistance or impedance and are essentially negligible. It is however possible, in the event that electronic equipment connected to the isolated system fails or causes an electrical short circuit, for a current path to develop between the secondary winding circuit and ground. Likewise if the electrical insulation in the transformer or in the electrical systme deteriorates over a period of time, a low resistance path to ground may be created. Obviously if one portion or side of the secondary lines are directly or indirectly connected through any resistance to ground then an electrical voltage exists between the remaining electrical conductor in that system and ground. If the impedance of the path to ground is sufficiently "low", current can flow. Thus should a patient be in contact to a grounded floor and be exposed to one of the other leads in the electrical supply system, the patient could sustain electrical shock.
Conservative practice dictates that the electrical condition of each side of the line be monitored. To actively monitor the degree of isolation of the electrical lines in an electrically isolated supply system, line isolation monitoring equipment is thus employed.
One apparatus for this purpose is described in U.S. Pat. No. 3,666,993 to Legatti, as has been made known to me, and I believe other similar apparatus exist. Present basic requirements for line isolation monitoring system are that the system be sensitive enough to detect a leakage on the order of 0.7 to 2.0 milliamperes of electric current, and, secondly, for the equipment itself to contribute a maximum leakage to ground of 20 microamperes, and, thirdly, for the maximum current under a test condition, in which each side of the line is short-circuit to ground, alternately, to be 1 milliampere.